Ed, Patrcik, Kiwamu,

In addition to the ISS issue (alog 22863), tonight we had two other issues with the alignment loops (DHARD instability and SRC2 instability, reported in alog 22859). After some investigation, we found that IM1 and IM2 had different alignment than usual, mainly in pitch. Even though we are not sure how exactly this affected all three issues we had tonight, we realigned them so that the OSEMs read the same values as the past. This resulted in two positive consequences as follows.

(1) Realigning IM1 and IM2 brought IM4 trans and POP_B QPDs back to where they were.

(2) The realignment increased the carrier power in both PRC and arm cavities by 5 or 6 %.

See the first attachment for the increased carrier power. We aligned IM1 and 2 while the interferometer was locked at low noise. We accepted the new IM alignments in SDF (see the third attachment). Even though we still suffer by the DHARD instability, at least the SRC2 instability seems to have gone away after the realignment. I did not attempt to maximize the carrier power but it makes me think that there might be a "sweep spot" for IMs' alignment where the carrier power is maximized in the interferometer. Also, note that IM4 trans sum increased as well with the newly aligned IMs, but only 0.6% perhaps due to the beam on IM4 trans getting centered.

IM1 and IM2 moved likely due to the untrip of HAM2

According to trends, it seems that both IM1 and 2 changed the alignment at the time when Ed untripped HAMs 2 and 3 (alog 22835). This effect is something we already knew (alog 22719). For some reason, this time it changed mostly IM1 and IM2 in pitch. See the second attachment for the trend. IM1 moved by about 1 urad, and IM2 moved by about 50 urad as seen by the OSEMs. Also, yaw of IM1 and 2 also moved, but neither IM4 trans nor POP_B see a significant change in yaw. The MC mirrors seems to have been unchanged.

DHARD is still unstable

I was hoping that the realignment of IMs would magically fix all the issues we had, but the DHARD instability seems to remain. The symptom is that both pitch and yaw DHARD oscillate at around 0.6 Hz. As was suggested by Jenne, we decreased their loop gain which fixed the instability. I have no idea why they oscillate at 0.6 Hz and why the gain reduction helped. The pit gain is set to 7 (which was 10 originally), and the yaw gain is set to 10 (was 15 originally). For this lock stretch, we accepted these values in SDF and this means that they will show up in the SDF next time when we lock the interferometer. See the fourth attachment for the SDF.

To those who will relock next time

Since we have aligned IM1 and IM2 in situ when the interferometer was locked at low noise, the interferometer alignment in the cold state will be somewhat far from the optimum. I would recommend running the full initial alignment before relocking.